Concrete-filled steel tubular columns are today a preferred choice of designers for high load carrying capacity per unit area of cross section of columns. Such columns provide faster construction and no requirement for framework. Patents are available giving descriptions of the methods of construction of such columns and cantilevers. However, the problem associated with this type of construction is in the escape of gases during exposure to relatively intense fires especially when the columns are massive.
This problem is exacerbated with the use of high strength concrete because of the reduced porosity thus providing fewer escape routes for gases during fire exposure. The mixing of polypropelene fibers in the concrete of such members helps to provide passage because of the fibers melting during the fire produce porosity for the escape of gases from inside the concrete mass. However, the mixing of polypropelene fibers in concrete will not be that effective because of the requirement of a large number of vents required in the steel tube which is not structurally feasible.
The exposure of such columns to fire may even lead to its bursting in the case of insufficient vents for the escape of gases. Most of the currently available studies use vents in the outer steel column for the escape of gases. A patent search on concrete filled steel tube columns disclosed the following patents.
A U.S. Patent of Sato U.S. Pat. No. 4,722,156 is entitled CONCRETE FILLED STEEL TUBE COLUMN AND METHOD OF CONSTRUCTING THE SAME. As disclosed, a concrete filled steel tube column includes a steel tube having an inner face; a concrete core disposed within the steel tube; and a separating layer interposed between the inner face of the steel tube and the concrete core for separating the concrete core from the inner face of the steel tube. Therefore, the steel tube is not bonded to the concrete core. After the separating layer is formed on the inner face of the steel tube, the concrete is charged into the steel tube to form a concrete core.
A further U.S. Patent of Schleich et al. U.S. Pat. No. 4,779,395 discloses a COMPOSITE CONCRETE/STEEL FIREPROOF COLUMN. As disclosed, a fireproof construction element has a plurality of integrally interconnected and parallel profile beams each having a longitudinally extending outer flange defining an outer surface and a longitudinally extending web extending inwardly from the flange. The webs are each formed adjacent the flange with a row of at least generally longitudinally extending, elongated, and laterally throughgoing slots. The beams form a plurality of outwardly open channels laterally bounded by the flanges. Respective masses of concrete substantially fill the channels between the webs and inward of the flanges and have outer surfaces contiguous with the outer surfaces of the beam flanges. The slots can be provided in two rows with the slots of one row overlapping and staggered with the rows of the other.
Finally, a U.S. Patent of Vincent U.S. Pat. No. 6,061,992 discloses a Composite Steel/Concrete Column comprising a longitudinally extending H-shaped steel assembly having a pair of parallel flange plates and a web plate interconnecting the flange plates and defining two opposite channel-shaped spaces. A plurality of spaced-apart transversal tie bars are disposed along the steel assembly on each side of the web plate for interconnecting the flange plates. A mass of concrete fills the channel-shaped spaces. The steel concrete column is characterized in that the ratio of the cross-sectional surface area of the steel assembly with respect to the total surface area of the composite steel/concrete column is less than 9%, preferably 2% to 5%. The column is principally to be utilized in structural steel high-rise buildings which have the advantage of shop prefabrication resulting in rapid on site construction. The column shows a steel to concrete ratio greatly reduced as compared to prior art composite columns, thereby greatly reducing the production cost and the size of the columns.
Notwithstanding the above, it is presently believed that there is a need and a potentially large commercial market for improved concrete-filled tubular steel columns for high load carrying capacity. It is also believed that the steel tubular columns for high load carrying capacity in accordance with the present invention can be produced at a competitive cost and yet possess the physical properties required in the building industry. Further, it is believed that if a fire is encountered, there are methods for retrofitting the columns at a reasonable cost.